In federated architectures, individual sensors, components, and devices may be clocked independently because their designs are optimized, or their development matured, separately from each other. As modern advanced distributed systems become more highly integrated, the need increases for information fusion of data originating with these federated sensors that requires both tight timing synchronization and frequency syntonization. For example, sensors in RF ranging/timing systems may require navigation-level timing synchronization at a precision level of 1 ns or less. Similarly, signal intelligence (SIGINT) applications may require the coordination of frequency syntonization to a precision level on the order of 0.1 ppb or less.
Conventional approaches to this problem include, for example, the use of Time Interval Counters (TIC) to measure the time difference between two events as defined by pulses originating from two independent sources. These pulses may be derived from a clock pulse but must be shaped with very sharp edges, and transported via broadband cabling, to preserve the high frequencies associated with such sharp edges. The high frequency content necessary to maintain such sharp pulse edges may extend considerably higher than the clock's own intrinsic frequency. In addition, to prevent ambiguity in the identification of a pulse, the pulses may be transmitted at a relatively low rate (e.g., once per second). However, the need to limit the pulse transmission rate may require excessive time to average out sampling errors. For systems requiring quick synchronization or that use more cost-effective and less stable clocks, such long averaging times may result in reduced relative timing accuracy. Furthermore, to achieve the desired performance (e.g., synchronization to within 10 ps) via TIC or other absolute timing based approached would require terahertz (THz, 1012 Hz) level clock signals. Terahertz signals are not supported by the traditional copper cabling installed aboard aircraft or other platforms, and would require a costly upgrade to fiber optic cabling.
In addition, the limit of the integer resolution of the local oscillator clock used to time the interval between two pulses generally requires a high sampling frequency for precise relative timing. To mitigate this need for high frequency references (e.g., 50 GHz for 20 ps), interpolation techniques may be employed. Interpolation may allow fractional clock cycle counting which increases resolution beyond that of the local clock. Time to Digital Converter (TDC) techniques can be implemented on a low-cost field programmable gate array (FPGA); the Vernier method may achieve uncertainly of <100 ps, and fully digital systems typically achieve 50-500 ps. However, the use of TDC techniques increases implementation complexity; furthermore, TDC techniques are limited to providing time difference information between input clocks and cannot provide frequency difference information.
Another approach, a Dual Mixer Time Difference (DMTD) system (including digital DMTD (DDMTD) implementations), may determine the phase difference of two clock signals of the same nominal frequency by mixing the two signals with an internal clock signal of slightly different frequency to generate a low-frequency output (relative to the clock signals) for analysis by a TIC. With respect to DMTD/DDMTD-type systems, traditional methods of disseminating time or frequency information (e.g., Network Time Protocol (NTP), Precision Time Protocol (PTP), Inter-Range Instrumentation Group (IRIG)) cannot provide the required precision and distribution for equipment incorporating a significant cable delay (e.g., 2 km). For example, DMTD/DDMTD systems may calculate cable delay from coarse offsets using time-tagged two-way messaging, and calculate fine phase offsets via DDMTD to syntonize system clocks to a master clock. While DMTD and DDMTD implementations may work well in controlled environments incorporating highly stable clocks of common frequency, many real-world airborne, vehicle-based, remote, or ground-station applications may not include sensors of similar frequency or consistent phase (due, e.g., to their cost). Similarly, such applications may not tolerate syntonization of their sensors (due, e.g., to the need to maintain fault independence).